Most existing treatments for strabismus rely on compensatory impairment of extraocular muscles (EOMs);we are developing an alternative to strengthen EOMs and alter their stiffnesses and lengths in controllable ways. Hypertrophy of fast muscle, such as EOM, is a consistent consequence of intramuscular injection of bupivacaine (BUP) in animals. BUP is a locally acting myotoxin that causes muscle fiber degeneration, satellite cell activation, and muscle fiber regeneration, continuing to hypertrophy. Inadvertent injection of BUP into human EOM during retrobulbar anesthesia causes a pattern of strabismus that modeling suggests is due to significantly increased contractility in the muscle's field of action and slightly increased stiffness affecting the opposite field. We also have preliminary results suggesting that muscles are regenerated at a length determined by eye posture during regeneration. This project aims to explore the types, amount, persistence, and mechanisms of changes in EOM after BUP injection by means of biomechanical and histological studies in rabbits, and by clinical, MR imaging, muscle force, and simulation studies in strabismus patients. We will inject BUP into normal and experimentally paralyzed rabbit eye muscles, with some eyes held in predetermined postures during regeneration. We will measure changes in muscle dimensions, elasticity, and contractile force. We will histologically assess muscle fiber counts, sizes and separations, and structural and ultrastructural changes, including scarring and fibrosis. We will vary injected volume and concentration to determine the dosage of BUP needed to safely produce beneficial effects. We have begun a pilot trial of BUP injection into EOMs of adult strabismus patients with very encouraging results in comitant horizontal cases. This will be extended to vertical and oblique muscles and to paretic disorders. We will use magnetic resonance imaging (MRI) before and at intervals after injection to verify injection sites and measure changes in muscle size. We will measure and compare binocular alignment, ocular motility, saccadic velocities, and muscle forces before and after injection, and note any side effects or complications. As data on dosage, injection technique, and safety become available and standardized, we will design a randomized trial comparing BUP injection and strabismus surgery for horizontal strabismus. Controlled muscle strengthening by BUP injection appears poised to provide a powerful and valuable new tool for strabismus management. Regenerating muscles at reduced lengths would mimic the effects of surgical resection. Regenerating muscles at increased lengths would produce effects unobtainable with surgery. Fully understanding the basic muscle changes in animal experiments and accurately documenting muscle size changes and function in patients with various disorders are both essential to this end. BUP injection also holds promise for treatment of eyelid ptosis and for muscle disorders beyond ophthalmology. PUBLIC HEALTH RELEVANCE: Injection of the local anesthetic bupivacaine can increase the size and strength, and alter the lengths of muscles. We will develop this finding as an alternative to surgical treatment for strabismus, or crossed eyes. Extension of this approach holds great promise for treatment of many muscle disorders.